Cons: Volume control not flexible in stock form but that's easily rectified

Fantastic amp. Easy and fun to build. Many opportunities to modify the amp. Vibrant on line community dedicated to the amp.

Due to it being an otl amp, it's optimized for high impedance headphones. Don't bother with Grados, for instance. I don't view this as a con, but merely as a design characteristic. Truly synergistic with my HD600's.

I've used film and teflon bypass caps to bypass all critical caps, and padded down the volume control as in stock form, it's way too loud by the 10 o'clock setting, also using beautiful NOS tubes. So no,,this isn't a stock Crack, but if you've got the DIY leaning, this is a great project.

This is a review of the Crack amp driving Sennheiser HD800s. I haven't had the chance desire to audition it with any other cans. The price is for the amp ($213) and the Speedball upgrade ($110).

I've had the combo for about two years, and have several hundred hours on it. The Crack is an OTL headphone amp kit, designed for high-impedance phones. Assembly is easy, the directions are excellent, and support on the Bottlehead forum from Dan Schmalle and his very active, knowledgeable customer base is superb. After living with it for a while, I added the Speedball power supply upgrade; it may have made a slight improvement - hard to say without being able to A-B it. I also replaced the stock 6080 tube with a WE 421A, which made a definite improvement - everything opened up more. If you go that route, be patient: these tubes come up pretty often for under $50, and sometimes half that, while there are always a few hugely overpriced examples available for someone in a hurry. Besides, the WE tube looks cool. I know, looks don't matter, but this tube, with its retro Coke-bottle shape and original "lightning" Western Electric logo on the base is just too good to pass up. I forget which 12AU7 I have and the writing is long gone - probably an old Mullard CV4003.

So how good is the combo? It is close to the best electrostatics driven by great tube amps. Mids and highs are superb: flat, extended, smooth, detailed, and never tiring. Bass is very natural sounding, and, thanks to the 800s enormous geometry, never gives that "pounding on your eardrums" effect I get from almost all other cans. Very deep bass is there when called for, with no trace of boominess. Electrostats will give that extra bit of "liquidness" but always at a cost - uncomfortable, hard to drive, you know the list. The Crack on the 800s gets you very close, and with a comfort level not matched by any other phones, at least on my head.

How much of the good stuff is due to the Crack? Quite a bit, I think. Listening through a BGW 100A amp, everything sounded just a little flat. The dynamics didn't bloom as naturally, though, typically for the old BGW stuff, its mids and highs were as smooth as any tube amp. I thought maybe the Crack was smearing things a bit, but that is not the case: recordings I'm very familiar with reveal all sorts of new detail with this combo, but in a way that's addictive. Example: intending to listen to a cut or two from the brilliant Speakers Corner re-pressing of Stevie Wonder's Songs in the Key of Life, I'm now finishing side four. And to those who say the HD800s or the Crack is bass-shy, listen to this record on good gear; you will never say that again. Oh... session files or tapes made with a real EMT 140 plate will give you goosebumps - you can almost "see" the waves traveling in the steel.

Recommendation: if, like me, you spend far too much time with headphones, add a Corda crossfeed circuit to the input side of the Crack - find the schematic for free on the web. There's plenty of room to add a rotary switch with the caps and resistors hung directly off it, but that gives you a rat's nest, so be neater than me and put everything on a separate board. The Corda circuit lets you dial back the separation, which makes the presentation of some material much more natural.

Pros: Perfect for my HD600, boosts volume/bass/soundstage, very quiet background and no distortion at high volumes, fun to build as DIY

Cons: Not many features, only for high impedance cans, aesthetics could be better

This was an amazing and fun purchase for me. It was fun to build and it makes my HD600s sound great. Very simple design that does everything I need it to for my headphones. I don't believe I'll ever purchase another amplifier unless I fall in love with a low impedance headphone that needs amplification.

Pros: Incredible value, great soundstage presentation, ability to adjust sound via tube rolling, get to build it and customise it for yourself

Cons: Can't use it straight from the box :-), may take some tinkering to get the sound free from induced noise, performs best with high impedance cans

The Crack kit is easy to build and sounds incredible. Don't be scared of the DIY approach even if you're a beginner - it's fun and SO worth it!

Mine had some induced noise at first so I replaced the braided input cable with some shielded coaxial cable and it fixed it perfectly. I see that and potentially damaged/faulty tubes as being about the only real possible issue with this amp.

Once built (takes a day or 2 depending on how long you spend in one sitting) the final result is an amp with incredible staging and layering for the price combined with beautifully smooth and cohesive sound presentation. There's plenty of detail, but it's all cohesive and realistic - it doesn't pull apart your music into individual pieces.

This amp easily outperforms significantly more expensive units and is hard to beat with >250 ohm cans like the top end Beyers and Sennheisers. It sounds so sweet with my HD650s.